?/DIV>
=
(20)
where 4.1 V is the control range of the error amplifier
and f
0
is the desired voltage loop crossover frequency.
It is important to consider that the lowest output ripple
frequency limits the voltage loop crossover frequency.
In PFC applications, that frequency is two times the AC
line frequency. Therefore, the voltage loop bandwidth
(f
0
), is typically in the 5 Hz to 15 Hz range.
To guarantee closed-loop soft-start operation under all
conditions, it is recommended that:
SS
HF
COMP,
C
4
C
?/DIV>
<
(21)
This relationship is determined by the ratio between the
maximum output current of the g
M
error amplifier to the
maximum charge current of the soft-start capacitor.
Observing this correlation between the two capacitor
values ensures that the compensation capacitor voltage
can be adjusted faster than any voltage change taking
place across the soft-start capacitor. Therefore, during
startup the voltage regulation loops response to the
increasing soft-start voltage is not limited by the finite
current capability of the error amplifier.
LF
COMP,
0
COMP
C
f
?/DIV>
2
R
?/DIV>
?/DIV>
?/DIV>
=
1
(22)
COMP
HFP
HF
COMP,
R
f
?/DIV>
2
C
?/DIV>
?/DIV>
?/DIV>
=
1
(23)
where f
HFP
is the frequency of a pole implemented in
the error amplifier compensation network against high-
frequency noise in the feedback loop. The pole should
be placed at least a decade higher than f
0
to ensure
that it does not interfere with the phase margin of the
voltage regulation loop at its crossover frequency. It
should also be sufficiently lower then the switching
frequency of the converter so noise can be effectively
attenuated.
The recommended f
HFP
frequency is around 120 Hz in
PFC applications.
Step 14
: Over-Voltage Protection Setting
(OVP)
OVP
LATCH
OUT,
OV2
P
V
3.5V
R
?/DIV>
=
(24)
where 3.5 V is the threshold voltage of the OVP
comparator and P
OVP
is the total dissipation of the
resistive divider network. Typical P
OVP
power loss is in
the 50 mW to 100 mW range.
OV2
LATCH
OUT,
OV1
R
1
3.5V
V
R
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
(25)
R
OV1
can be implemented as a series combination of
two or three resistors; depending on safety regulations,
maximum voltage, and or power rating of the selected
resistor type.
Step 15
: Input Line Voltage Sense
Resistors
INSNS
MIN
,
LINE
MAX
,
LINE
2
IN
P
V
2
V
V
925
.
0
R
2
"
"
"
=
(26)
where 0.925 V is the brownout protection threshold at
the VIN pin. V
LINE,MIN
is the minimum input RMS
operating voltage. Its divided down level at the VIN pin
corresponds   to   the   0.925 V   brownout   protection
threshold. V
LINE,MAX
is the maximum input RMS voltage
anticipated in the design and P
INSNS
is the total power
dissipation of the R
IN1
- R
IN2
divider when the input
voltage equals V
LINE,MAX
. Typical P
INSNS
power loss is in
the 50 mW to 100 mW range.
IN2
MIN
LINE,
IN1
R
1
0.925V
V
R
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
2
(27)
R
IN1
can be implemented as a series combination of two
or three resistors; depending on safety regulations,
maximum voltage, and or power rating of the selected
resistor type.
糀
2
V
.
0
R
R
R
V
2
R
IN2
IN1
IN2
ON
LINE,
INHYST
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
+
?/DIV>
?/DIV>
=
925
(28)
where 0.925 V is the threshold voltage of the line
under-voltage lockout comparator and 2 糀 is the sink
current provided at the VIN pin during line under-
voltage   (brownout)   condition.   The   sink   current,
together with the terminating impedance of the VIN pin
determines the hysteresis between the turn-on and
turn-off thresholds.
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